Method of and apparatus for controlling systems



Examiner w 252 Ex Reference c. o. FAlRcHlLD" METHOD 0F AND APPARATUS FORCONTROLLING SYSTEMS Filed March 19, 1937 FIPszlz Dec. 23, 1941.

5&5

ATTORNEYS NWI a y xm mi bw Nk Patented Dec. 23, 1941 METHOD OF ANDAPPARATUS FOR OQNTRQLLING SYSTEMS Charles 0. Fairchild. St. Albans, N.Y., assignor to Charles J. Tagliabue Mfg. Co., New York, N. Y., acorporation ol' New York .f

Application Much 19, 1937, Serial No. 131.843

24 Claims.

This invention relates to methods of and apparatus for controllin:systems particularly of the type, described in my Patent 1,970,559,which include a light source, a phototube (or other photoelectricreceiver), a mirror galvanometer, and associated controlling circuits.This application is a continuation-impart of my U. S. Patent No,2,205,777, granted June 25, 1940.

A particular object of the present invention is to provide a controllingsystem whichisosimpler. less expensive, more sensitive. and morereliable than other systems which have been heretofore described for thepurposes for which this invention is suitable. This invention dinersfrom the one described in my patent mentioned above in the absence ofperiodic and intermittent exposure means operating between the movingmirror and photoelectric receiver and is characterized by a fasteraction and more sensitive controlling ability.

It may also be mentioned that it has heretofore been proposed to provideelectrical remote control or follow-.up systems in which the controlledelement is moved at one rata of speed when there is a large disagreementbetween its position and the position of the control element, and at aslower rate of speed when the positions of the control and controlledelements are approaching correspondence. The present invention does notreside in such a two-speed follow-up system but is characterized by thefollowing features.

According to the invention there is provided an electrical controlsystem in which a motor for restoring a balancing system to equilibriumis controlled by relay means in a governing circuit, and a measuringelement sensitive to departure of the balancing system from equilibriumcontrols the relay-energizing current in the governing circuit, the rateof restoration of said current to a value corresponding to equilibriumin the balancing system, following a departure from said value, beingdetermined by one substantially fixed time-constant when the current isfar from said equilibrium value and by another higher substantially xedtime-constant when the current is near said equilibrium value, i. e.near 0r in a dead-zone current range for the relay means.

The invention also resides in an electrical control system in which amotor for restoring a measuring circuit of the balancing type toequilibrium is controlled by relay means in a governlng circuit, and emeasuring element such as a galvanometer sensitive to departure of themeasurine circuit roin equilibrium controls the relay-enersirlinscurrentmittig gggermng circuit.

the rate of restoration of said current to a value corresponding toequilibrium in the measuring circuit, following a departure from saidvalue, being determined by one substantially fixed timeconstant when thecurrent is far from said equilibrium value and by another highersubstantially fixed time-constant when the current is near saidequilibrium value, l. c., near or in e dead-zone current range for therelay means.

The invention also resides in an electrical control system in which amotor for restoring a bal.. ancing system to equilibrium 1s controlledby two relays ln a governing circuit, the closing current value of onerelay being lower than the closing current value of the other relay. andthe arrangement being such that. when both relays are open, therestoring motor will be operated in one direction while, when bothrelays are closed, saidmotor will be operated in the other directionand, when one relay is closed and the other open (i. e. when the currentis in a dead-zone range) the motor will not be operated, and in which a.measuring element sensitive to departure of the balancing system fromequilibrium controls the relay-energizing current in the governingcircuit, the rate of restoration of said current to a valuecorresponding to equilibrium in the balancing system, following adeparture from said value, being determined by one substantially fixedtimeconstant when the current is far from said equilibrium value and byanother higher substantially fixed time-constant when the current isnear said equilibrium value, i. e., near or in a dead-zone current rangefor the relays. Preferably the time-constant of the governing circuit isincreased in the dead-zone range by means lncludins a capacitor in theenergizing circuit o! the relays.

According to a preferred form of the invention, the balancing system isof the Wheatstone bridge or potentiometer type and a sliding contact ismoved in one direction or another by the governing circuit under controlof a galvanometer to bring the value of an adjustable E. M. F. intocorrespondence with the E. M. F. of a thermocouple which changes inaccordance with the value of a main variable.

In one form of the present invention, the beam of light from the movingmirror is focused upon the plane of a controlling edge of thephotoresponsive receiver and Indies in correspondence with variations ofa controlled quantity. stably swinging to and fro across the edge and,through its effect on the receiver, operating relays which .n 1*. we

control a reversing motor, this motor being used for driving anysuitable control device.

1n the photoelectric circuit described here, the phototube is not acalibrated element but serves only to detect the displacement of alight-beam with reference to the controlling edge and in co operationwith the rest of the circuit to drive the controlled reversible motor inone direction or the other or to hold the motor against rotation whenthe light-beam is in a normal position corresponding to no deflectionfrom its so-called zero position. It will be evident from the followingdescription that a new and improved method of control is provided whichpermits relatively rapid action of the reversing motor and at the sametime provides a higher sensitivity.

In order to accomplish such a performance in a particularly simple waythere are included in the photoelectric circuit two relays, one of whichis adjusted to close at a dierent value of current from that required bythe other and in which also are included means for delaying the startingor succeeding reversal of the reversing motor upon a deflection of thelight-beam. There are also included in the control system one or moredevices for bringing the moving mirror into phase with an oscillatingvariable quantity or to advance its phase ahead of that of the variable.

In a particular application in which the system is used for a recordingdevice, the reversing motor may serve to adjust an electrical quantityto equal the corresponding value of a measured variable as it has beenconverted into an electric quantity, the mirror being attached in thiscase to the moving coil of a galvanometer which serves to indicate thedifference between the variable and the adjustable electric quantityand. in particular, to indicate equality when the lightbeam nds itsnormal or zero position.

Connections between electric contacts on the two relays and thereversing motor are such that when one relay is closed and the otheropen the motor stands still and when both relays are closed or bothopen, the motor runs one way or the other. Both relays will be open whenthe light-beam does not shine upon the receiver and both closed when allof the beam or a sufiicient part of it does shine upon the receiver andone relay will be closed and the other open when a smaller fraction ofthe beam falls on the receiver, the beam in this case being split by thecontrolling edge. The cross-section of the image of the source in theplane of the edge in a direction perpendicular to this edgc is keptsmall, but may be adjustable for reducing sensitivity as in myaforementioned U. S. Patent No. 2,205,777.

Before proceeding with a description in detail of the ngures shown andthe mode of operation of this system, it will be necessary to callattention to certain characteristics of moving-coil galvanometers. It isuseful in describing such a controlling system as this to consider it inan oscillating condition and to consider the phase relation of variouselements. In this particular system it will become apparent that theforces applied to the reversing motor are not in proportion todeflections of the galvanometer but take definite values as the relaysoperate in the manner described. It is hence apparent that in the mannerin which this system might oscillate, there is no motion of a simpleharmonic nature. Let us suppose for example that the reversing motor hasreached full speed in one direction and that it is rapidly changing thecurrent through the galvanometer at a constant rate. In this condition,the galvanometer develops a constant back E. M. F. proportional to itsconstant angular velocity and lags behind the motion oi the motor by anamount measured by this back E. M. F. It has been discovered that thislag is reduced to zero when an auxiliary, advancing E. M. F. is appliedto the galvanometer in series with the normal E. M. F. and equal andopposite to the generated back E. M. F. 1f the galvanometer is movingtowards its normal position, then, when the light-beam reaches thenormal position the motor is also at a corresponding position and, ifboth could be stopped suddenly, a correct control condition of thesystem would then be reached. If, however, the advancing E. M. F- islarger than the back E. M. F., the galvanometer leads the motor and thelatter is stopped before reaching the normal position. Now, if thisadvancing E. M. F. has been applied through contacts arranged on therelays, it can be removed at the moment the first of the relays operatesand this will cause an impulse resulting from the still unbalancedquantities to act against the tendency of the galvanometer to swingbeyond the normal position because of its inertia. In fact, theadvancing E. M. F. can be so large that by its removal the galvanometerwill be kicked back from the normal position, as a result oi which therelay which is operated will reverse its own action and the motor andgalvanometer will proceed to take short steps towards the normalposition at a rate depending upon the time-constant of the relay and itsassociated electric circuit.

It is apparent that owing to the presence of various inertial forces,the best value of the advancing E. M. F. is readily determined,following my above teaching, by experiment and will take into accountthe rapidity of action of the relays, the overrunning of the reversiblemotor, the operating characteristics of the galvanometer, and the scalerange of the recorder. Suppose, for example, that a recorder has a fullscale range of 10,000 microvolts traversed in 50 seconds at full motorspeed and that it is necessary to determine a balance or to measure thevariable quantity within 0.02% of full scale, corresponding to 2microvolts, a span traversed in 0.01 second. To stop the motor in soshort a time as this would ordinarily require power far in excess ofthat normally available in an instrument or controller of the type inmind. Moreover, a galvanometer sufiiciently sensitive to serve for thispurpose would have a back E. M. F. of, e. g., 50 times this value of 2microvolts at the angular velocity corresponding with the speed oftraverse mentioned, such that without the advancing E. M. F. mentionedabove the galvanometer would be lagging by an amount far in excess ofthe required accuracy.

Another main object is the alteration of the time-constant of responseof the instrument as the galvanometer traverses the limits of itsdeadzone.

Thus, the chief objects are to provide methods and means for causingsuch a sensitive galvanometer to stably return to a definite balancingpoint and, in particular, by in eect temporarily shifting the balancingpoint upon a galvanometer deflection.

These and such other objects of the invention will appear to thoseskilled in the art from the BCCOmDBDying drawing and specincation, inwhich is illustrated and described a specific embodiment of.theinvention. It is my intention to .ddahnalithatlhavediaclosedthatisnewand Fig. ladiagramoianelectricalmeasuring circuittogether with the photoelectric circuit with amplier. relays and lamp.with which it cooperates.

Fig. 2 is the preferred embodiment of part of the system shown in Fig.l.

Fig. 3 is a graph showing both beam position and plate current plottedagainst time.

Referring to Fig. l, the temperature of thermoconple 21 is measured bythe position of sliding contact 4l along its slide-wire I8. When thesecorrespond, the reflected light beam from the mirror of themirror-galvanometer is in the normal position as shown so that it ispartly on and partly ofi phototube 8|, the light beam being split by theshielding edge 8 l In this normal position of the light beam, relays Iland l2 are respectively closed and opened as regards the amaturepositions. Either relay is said to be closed when its armature is in theposition closest to the relay coil and open when it is in its iurthestposition away from the coil of the relay. Some of the relay contacts areclosed in the extreme positions of their armatures and are open duringthe instant in which the armatures are passing from one extreme positionto the other. Each oi the coil armatures is continuously biased awayfrom its respective coil so that when the value of the current is abovea predetermined value for that relay the armature is closed and one setoi' contacts is eiiective and when the current value drops much belowthe predetermined value, the biasing force holds the armature away fromthe coil so that the other set oi' contacts is closed.

Upon an increase, e. g., o! the temperature o! the thermocouple, thegalvanomeier swings the beam in a clockwise direction to decrease theillumination of the phototube and hence to decrease the relay current toincrease to cause relay I2 to close which causes the reversing motor tooperate to move the sliding contact upscale, i. e., to the right inFig.l.

The potentiometric circuit A potentiometric circuit is shown in whichthe ir drop in slide-wire I8 provides a potential gradient against whichthe potential due to the tact 28 being moved from the normal operatingposition shown in which contact 26 touches con tact 85 to the oppositeposition in which contact I8 touches contact 21. In case the calibrationis found not to be correct, adjustable resistor I8 is moved by theoperator to a position to bring the galvanometer 28 to its normalposition when the sliding contact 4| is at a predetermined position onits slide-wire I8, preferably at come selected point near the middle ofthe scale.

Starting with switch 84 in its usual operating position in which contact38 touches contact 8i and with a balanced condition of galvanometer 88,the position of sliding contact 4l corresponds with the temperature ofthermocouple 21 and motor II2 is stationary. In this balanced condition,the positive side oi' the thermocouple 21 is connected with thegalvanometer and the negative side oi the thermocouple is connected withpoint 28 of the potentiometric circuit.

Upon a decrease of the temperature of ther- 2 'Crfi's's- Referer.Examine* 25 o 2 O 1 aaeasei 3 mounle 21 the mirror nlvanometar turna ina counterclockwiso direction to increase the amount of light rchim thephototuhe and may reach astop N'whichissoastokeepanyoi the beam fromgoing oil' the cathode 85 ci photombe 8|. 'Die increase of illuminationo! the phototuhe us an increase in the current in relays H andl2sothatrelay l2alsoclosesthus shifting the point ci connection of thenegative side oi thermocoupie 21 from point 28 o! the bridge to point 83since the negative aide or the thenncccuple is connected with point ilby wire 32, resistor 8l, wire 38, contacts 41 and 48 (which are closedwhen relay H is closed), wire 88, contacts $8 and 49 (which are closedwhen relay l2 is closed), and wire 82 to point i3. Point 8l has a higherpotential than point 28 due to the ir drop through resistor 22, theoperating current being provided by battery I3. Consequently a shift cithe connection oi the negative side of thermooouple 21 from point 28 topoint 53 lowers the eiiective potential and the current in galvanometer88 so that the shift of the point oi connection from 28 to I8 may besaid to oppose the change in temperature of the thermocouple 21.

In case o! an increase of the temperature o! thermocouple 21, thegalvanometer turns the beam clockwise so that it passes from thephototube onto the shield whose edge is il' so that no direct light fromthe reflected beam reaches the phototube. In this condition, the currentdrops in the coils o! relays Il and l2 so that both relays open and thenegative side o! thermocouple 21 is connected with a point 88 oi lowerpotential than point 28 due to the ir drop in resistor 23. Following thecurrent from the ncsative side o! thermocouple to point 58. the cun'entiiows through wire 22, resistor 3|. wire 30. contact: 4l and 48 (whichare closed when relay Ii is open) and wire I4 to point 88.

Following the current around from the positive aide o! thermocouple 2l,it ilows through wires 23 and contacts 85 and 38 of the manual switch24, through the coil o! mirror galvancmeter 28 and through wire 28 tothe return bar 48 against which sliding contact 4I bears. as well asagainst sliding wire i8. The current returns through resistors 28, 24and 23 to point 28 on one side of the potentiometric bridge and throughresistors l1, 2l and 22 on the other side to point 28. From point 28,the current returns .through line 28 by contacts 8| and 84 (which areclosed since relay l2 is open). wire 88 and contacts 48 and 41 (whichare closed since relay Il is closed), wire 88, resistor 8l. and wire 32to the negative side o! thermocouple 21. A more precise description cithe potentiometric circuit follows: The potentiometer circuit isgenerally similar to that shown in my aforementioned U. S. Patent2,205,777 and in which current from battery l2 tlows through resistors2l. 22. 28 and 24, then to resistor 25, and conductor 28 back to i2. Asearlier described at greater length. when the E. M. F. oi thethermocouplc 21 is not balanced by the potentiometer, current will alsoflow from the connection 28 through conductor 28. the contacts of relaysIi and I2 when the former is held closed and the latter open, throughconductor 20, resistor 3|, conductor 32. thermocouple 21, conductor 32,manually-operated switch 84 in the position shown, contacts 85, I6, themirror galvanometer 38, conductor 88. the return bar 48 and thetravelling contacts 4I (which may carry an in.

dicating and/m'- rocordin eltmtlitl to the then 4 contact point 42 onthe slide-wire Il. The current through the slide-wire is standardized ina conventional manner by throwing the switch 34 to the reverse directionconnecting the standard cell 43 opposed to the battery. from theconnection 44 through resistor 45, contacts 21, 36 o! switch 34.galvanometer 38. conductor 39, bar 40, contacts 4I to the point 42 onthe slide-wire. when 42 is at a predetermined position selected forconvenience, as also explained in my U. S. Patent No. 2,207,343. Whenboth relays are closed current can now through the galvanometer 39,switch 24, thermocouple 21, resistor 3i, contacts 46, 41 on relay II,contacts 49, 50 on relay I2, conductor i2, to connection 53, and whenboth relays are open connection is made from 28 through contacts 41, 48on relay II, conductor 54 to connection 55.

Thus when the potentiometer is balanced against the thermocouple E. M.F. and relay II is closed and relay I2 is open, the point 42 isconnected through galvanometer I9, switch 34. thermocouple 21, resistor9|, contacts 46, 41 on relay Il. conductor Si, contacts SII, 5I on relayI2, conductor 29 to connection 29. A scale (not shown) for theslide-wire IB is calibrated with reference to the potential of the point28, the normal connection in the balanced condition. the cooperatingresistors (in particular. resistors 20, 29 and 24) being selected tohave the scale zero correspond with the desired potential. It isapparent then that when the potentiometer is not balanced that anauxiliary E. M. F. in one direction or the other corresponding to the ifdrop in 23 or in 22 will be imposed on the galvanometer, the directionbeing chosen as described later.

Preferred and simplified connections are shown in Fig. 2, in whichconductor 30 is permanently connected through conductor 29 to connection29, and the relay contacts are reduced in number. Fig. 2 is for thenormal condition in which relay II is closed and relay I2 is open. theoperative being such that blades 41 and 50 are respectively movable bythe armatures of relays II and I2. In Fig. 2, a downward movement towardthe relays results when the relay coils are sui'ilciently energized topull down the blades 41 and 50 which are continuously springbiasedupwardly, i. e., away from their respective relays. When both relays areopen, contacts 41. 48 are closed but contacts 49, 50 are open. When bothrelays are closed. contacts 41, 49 are open but contacts 49, 50 areclosed. Auxiliary E. M. F.s are obtained from the far connections ofresistors 24 and 2I through resistors 51 and $8 which are connectedrespectively to contacts 49 and 50 through conductors 59 and i0. In thisiigure the potential of 28 is altered by shunting resistor 24 withresistor 51 through conductors 59 and 29 and contacts 41 and 48 whenboth relays are open, or by shunting resistor 2I with resistor 58through conductors i0 and 29, and contacts 49 and 50 when both relaysare closed. By properly choosing the values of resistors 61 and 58 theshift of potential of 2l can be made equal to the value of the auxiliaryE. M. F. required. and in this case. as in Fig. 1, the auxiliary E. M.F. will be to a rst order of approximation, independent of the positionof the contacts 4I connected to the slide-wire I9 at 42. Provided theresistances of I! and the shunt I9 are not too large in comparison withthe resistances of 3|, 21 and 3| the effect o! this E. M. F. on thedeflection oi the galvanometer will be sui'llciently constant while 4Imoves across the slide-wire, being least when 4I is in the middle andgreatest when 4I is at either end. It will be shown that it is notessential to have an exactly constant effect, in subsequent discussion.

The governing circuit As earlier noted, a counterclockwise movement ofthe reected beam from the mirror galvanometer 38 causes an increase ofthe reflected light on the cathode i5 of phototube GI and in turn causesan increase in the current of the relays II and I2 which, starting withthe beam in its normal position. causes relay I2 to close when theillumination increases due to a decrease in temperature of thermocouple21.

Again referring to Fig. 1, this shows the balancing power-circuit orgoverning system, which is electrically independent of thepotentiometercircuit, and includes the phototube Si connected with itsanode i2 through conductor 63 to the plate 64, and its cathode 65through conductor $6 to the grid S1 of a triode 68, in the plate circuitof which are included the coils 69 and 10 respectively of relays Il andI2. A transformer 1I is used to supply the various voltages of thiscircuit, no auxiliary rectifiers being used for the two tubes.

The grid i1 of the tube 69 is connected through conductors 86 and 12,through resistor 13 and adinstable capacitor 14 which are in parallel,and through conductor 15 to what is commonly called the negativeterminal of the secondary of the transformer 1I. a useful convention inthis A. C. circuit which lacks a separate rectifier but has thephototube and the ampliiier more effective in each half-cycle in whichthe potentials are as shown than in complemental half-cycles. Filament19 of tube 68 is connected through conductors 11 and 18 to a lowdifferential voltage portion of the secondary. The lamp 19 is alsoconnected to a low differential voltage section in the secondary throughconductors 'l1 and 80. The plate 44 is connected to the plus terminal ofthe secondary through conductor BI, coil 69, conductor I2. coil 10 andconductor 93. A synchronous motor I4 may be connected if required. asfor driving a record chart, to the transformer through conductors l5, 9Sand 18. A stylus, pen, or other indicating or marking means (not shown)is movable with sliding contact 4I across the chart which is moved atconstant speed by motor 84 or along an indicating scale (not shown) asis well known in this art.

The thus-described phototube-amplifier circuit has been used before.However, it is believed to be novel and useful to provide such a circuitwith means for altering the time-constant of the circuit to stabilizethe balancing of the entire instrument, including the galvanometer,sliding contact, and the governing system itself. An auxiliaryconnection from the grid is made through conductor 91, capacitor 88,resistor 89. conductor 90 and contacts 9|, 92 on relay I2 when this isclosed, conductor 93. 4contacts 94. on relay I I when this is closed,conductor 96 to resistors 91, 98 bridging a portion of the secondary andconnected to it by conductors 15 and 99 and constituting asemi-adjustable potentiometer across this portion. When capacitor 88 isconnected in the grid circuit, the time of response and the timeconstant of the circuit are greatly increased or, in other words. therate of response is much lower.

It may be permissible to briefly describe the LUUITIAQ action of acapacitor under transient conditions for the sake of those who are notactivdy working in this art. For such. it may be desirable to clear up acommon misconception which is due to the occasional use of an incorrecthydraulic analogy, i. e., considering a capacitor as corresponding witha liquid reservoir in which the head or potential corresponds with astored current which is a diilerence between the iniiow and outowcurrents. The following brief note on hydraulic analogies may also behelpful in making it easier for equivalents to be set up in other thanelectrical circuits, e. g., in hydraulic governing systems and the like,in which the invention may be used with the flow of any governing iinidoccurring in the same sense generally as that disclosed for anelectrical current flow.

Actually, where there is a flow of electrical current (following theusual convention) into a capacitor, there is at each instant anidentical outward tlow so that there is in one sense no storage" ofcurrent in a capacitor as there is in a liquid reservoir. The storage ina capacitor occurs by putting the dielectric under a state of tension, ausefully-accurate hydraulic analogy of a capacitor being a closedchamber with a transverse resilient diaphragm through which the watercannot pass. Upon a current of liquid flowing into such chamber on oneside of the diaphragm, there is an equal outiiow from the chamber on theother side of the diaphragm at each instant with a gradual increase ofthe pressure which is necessary to cause a further displacement andstretching of the diaphragm.

If a shunting capillary (resistor) be provided from one chamber 'to theother, there will be a ow in the resistor due to the diterence inpressure across the diaphragm so that the pressures or potentials onboth sides of the diaphragm will iinally approach the same value. Justas in the case of an electrical capacitor with a shunting electr-lealresistor, such a resistor-shunted capacitor in a fluid system has adefinite and fixed time-constant with the characteristic, that the rateof change of the potential (pressure or voltage) is proportional to thevalue of the potential or strain (in diaphragm or dielectric).

Start, in a hypothetical case, with the mirror of galvanometer 8B insuch a position as would direct a reflected beam to be full on thephototube if the light 'I9 were on, but with the light 19 out, i. e.turned ofi. Then, after the light has been out long enough for thegoverning circuit to reach equilibrium and for both relays to open, thelight 19 is suddenly turned fully on.

The flow of current from the phototube 6i instantly reaches a valuecorresponding with the full amount of illumination. The grid potentialchanges at a high rate which depends upon the small capacity of I4 andthe resistance of 13 and the circuit has a denite but very smalltimeconstant, which means that the time of a given current response isvery short.

Then relay H closes to introduce capacitor Il and its series-resistor 89into this circuit which greatly lowers the rate of response and henceincreases the value of the time-constant, which means that the time of agiven current response is greatly increased. In other words, aconsiderable time is required before relay I2 can close. This time issufcient to permit the galvanometer to return from a ballisticover-swing and for the relays themselves to act.

I'he photoelectric current divides through resister I3 and capacitor 14which are in parallel.

Examiner If the photoelectric current now change is instantaneous andthe initial impedance of capacitor I4 is xero. the grid potential wouldbe instantaneously altered at the start. Instead, said 5 potentialbuilds up steadily but very rapidly as the charge builds in the smallcapacitor M which in turn provides a laurent-driving potential acrossresistor 73.

As is well-known in this art, the value of the l0 tlme-constantls simply1.- u TC in the equation for the change o! gn'd potential e=eo(l-e)where r is the resistance, c is the capacity. en is the ultimate valueof the change in the grid potential, c is the Napierian base, and t isthe time.

When the second capacitor 88 is connected to the circuit it chieflyaifects the value of the timeconstant as earlier mentioned. Theforegoing brief description is intended to show only the Purpose andfunctioning of this operation oi the governing circuit which provides adouble timeconstant o! the governing circuit.

In the photoelectric circuit oi Fig. 1, the two relays ii and l2 may bediicrent or alike in style but in either case relay il will be adjustedor selected to close at a lower current than relay l2 and thelatterwillopen ata higher current than that which causes relay il to close. If therelays are identical, the operating current of relay l2 can be raised byshunting coil lll with a resistor I, Resistors iM and I. are providedfor tbe purpose .of equaliaing the opening and closing currents of theirrespective relays il .and i2 which otherwise would be troublesomeiyaepuated. When relay Il closes, its coil 69 is shunted by resistor Illthrough contacts 402, |03 and when relay i2 closes its coil 'i0 isshunted by resistor i through contacts |05, 06. In both cases. theresistors I 0l and l are selected so that the releasing currents of therelays will be close to their operating or closing currents. 5 In orderto prevent the relays from chattering because of the half-waverectification of tube I8. coil I9 is shunted by capacitor lll, and coil38 is shunted by capacitor I8 and the two coils in series are .shuntedby capacitor |09 connected to conductors Hl and lli. These threecapacitors serve to smooth out the intermittent direct current and thevariable magnetic flux in the relays.

Still referring to Pig. 1, the negative bias of the grid i1 is soselected that with no illumination falling on the phototube il the platecurrent'will be well below the releasing or opening current .of relayil. With the phototube connected as shown with its anode to the plateand cathode to the grid, an increase of illumination will permit moreelectrons to ow from the grid thus decreasing its negative bias andconsequent- 1y increasing the plate or relay current. As theillumination and the relay current increases, first B5 relay H Iwillclose which will shunt its coil 69 with the resistor -IM as describedand the current through coil 88 will be reduced at this level ofillumination to a current value which is slightly above that for openingthis relay but the plate current will be increased as the externalimpedance of the plate circuit is decreased. However the plate currentwill not increase to a value near the closing current of reiay I2 unlessthere be a further increase of illumination. With a further U5 increaseof illumination and plate current, relay 12 will close, :hunting itscoil 18 and parallel resistor I 88 with an additional resistor I throughits contacts H and |86, and as this relay closes the plate current` willagain rise without further illumination ci the phototube as theimpedance of the external circuit is reduced by closing contacts IUS,|86. At this level o! illumination, however, the current through coil'Il is reduced to a value near the releasing or opening current of relayl2. But both relays stay closed as long as the level of illuminationremains thus high. Upon a fall of illumination, relay I2 opens first andthen relay II opens as the illumination further decreases.

Suppose now that the illumination of the phototube starts at a low levelA (see Fig. 3) such that both relays are open and increases at a uniformrate, that the photoelectric current is proportional to theillumination, and that over the range of illumination used the mutualconductance of tube 88 is constant, then as the illumination increasesat a constant rate the plate current will increase at a constant rateprovided these rates are sufiiciently slow. Considering for the sake ofclarity that the circuit is operated with constant voltages (D. C.)instead of alternating, following the convention shown on Fig. 1, andconsidering now that the rate of increase oi illumination issuiliciently high, then the timeconstants of the plate and grid circuitsand relays iniluence the rate of rise of the plate current. The rise ofplate current will be retarded somewhat by the high inductance of therelays but not appreciably by any lag in voltage of the grid, thenegative bias oi which will increase very nearly in phase with theillumination, provided the capacitor 'I4 and the tube capacitances arexuillciently small and the change of grid current with plate current isnegligible.

When the plate current reaches a value sufcient to close relay II,contacts 84, 95 will close and the capacitor 88 will be connected in thegrid circuit through contacts 8l, 82 which are closed, because relay I2is open. and through conductors I8 and 88. At this moment, the rate ofdecrease of grid bias is abruptly altered to be greatly retarded if thecapacitance of 88 is of a higher order than that of 1|. The capacitanceof I8 is so chosen that the rise of plate current will be so retarded asto delay the closing of relay I2 for an interval large in comparisonwith the normal operating time of the relays. 'Ihe values oi' resistors88, 91, and 98 are chosen so that, for the normal position of the lightbeam as shown in Fig. 1, the grid voltage will not be changed bymanually closing relay II while relay I2 is open. When nally the platecurrent has risen to a value sumcient to close relay I2, the capacitorI8 is removed from the grid circuit by the opening of contacts 8|, 82 byrelay I2 so that, when the illumination and plate current are decreasingfrom a higher value, relay I2 will open as promptly as possible.Likewise as before when relay I2 opens. capacitor 88 is again insertedwhich delays the opening of relay II upon a steady decrease inillumination. 1n other words, the relays act to insert capacitor 88 tonearly paralyze the grid circuit when the relay current is oi' such astrength as to indicate that the galvanometer is near its balancedposition.

The motor circuit The condition of the relays shown on Fig. 1, i. e.relay II closed and relay I2 open, indicates the balanced condition ofthe galvanometer 38 which in turn normally accompanies a correspondenceoi the position of sliding contact 4I and the temperature ofthermocouple 21.

In this balanced condition, contacts IIS, IIB are closed since relay IIis closed so that shading coil III is energized and contacts IIS and |20are closed (since relay I2 is open) thus likewise energizing coil Illwith the result that the motor I l2 is electrically locked or braked tostay in its last-set position. Upon a decrease of the temperature ofthermocouple 21, the reflected light beam from the mirror galvanometer38 moves in a counterclockwise direction to increase the light on thephototube and hence to increase the relay current so that relay I2 alsocloses.

The primary purpose of the relays is to operate the motor II2, indicatedas a shaded pole motor having a main coil connected to the line andhaving two shading coils H3 and III for reversing the motor. Contact IIBon relay II and contact IIB on relay I2 are connected through conductorII'I to both coils H3 and II4 of the motor. Contact II8 on relay II isconnected through conductor IIS to coil IIJ, and contact |20 on relay I2is connected through conductor I2| to coil Ill.

Description of operation The following rsum of the principal parts oithe electrical circuits and their functions is included here for reacLvreference in reading the appended claims.

A. Conventional potentiometer circuit-The slide-wire potentiometercircuit of Fig. 1 includes a thermocouple 21 and mirror galvanometer 38normally connected in series between the movable contacts 4I and a fixedselected point 28 in this circuit.

B. Means for advancing the phase of the galvanometer.-This point is oneselected from three iixed points, 55, 28, and 53 of diiierent potentialat the terminals of resistors 22 and 23, the selection depending uponthe position oi the relays II and I2 in the balancing power circuit.When a balance has been obtained (the galvanometer is at its normalposition with no current flowing through it). the connection is to point28 and the shifts to points or 53 are for the purpose of applying anadvancing E. M. F. to the galvanometer as it returns, from a swing awayfrom the normal position in either direction, towards the normalposition at which it reflects a beam o! light towards the controllingedge 6I' of the phototube 6I. An alternative circuit for the applicationoi' an advancing E. M. F. is shown in Fig. 2.

C. Method of stopping the matan-1n the potentiometer circuit of Fig. 1,the potential difference between the moving contact 4I made with itscontact-point l2 on the slide-wire and the fixed point 28 in the otherbranch is balanced against the E. M. F. of the thermocouple 21 by thereversible motor II2 with its two shading coils IIJ and Ill, either oiwhich is shortcircuited to start and drive the motor and both oi whichare shortcircuited to stop the motor, the connections of these shadingcoils being changed by the operation of the relays.

D. Amplifier part of balancing power circuit.- The balancing powercircuit or governing system is not conductively connected at any pointto the abovementioned potentiometer circuit. The balancing power circuitincludes the transformer secondary, the ampliier tube 88, the lightsource 19, the phototube 8|, the two relays II and I2 naving contactsinsulated from this circuit but connected into the potentiometer circuitfor shifting the connections to points 55, 28 and 83 therein. andcontacts in the balancing circuit for operating the reversible motor.

E. Means for rctardna the relay action-The balancing power circuit alsoincludes contacts 8|, 82, 84 and 85 for connecting the capacitor 88 whenrelay II is closed and relay I2 is open to retard changes in thepotential of the grid and hence of the plate current and of the relaysthemselves.

Considering the actual case where the temperature of thermocouple 21 hasbeen sady for at least a long enough time for the entire instrument toreach a condition of equilibrium with the slider 4I at a position onslide-wire I8 corresponding with the value of the temperature of thethermocouple, the mirror of galvanometer 88 is in the position to causethe light beam to split the phototube-shielding edge SI' as shown inFig. 1 and with relay I| closed and relay I2 open, closed and openedreferring, as earlier mentioned, respectively to near and far positionsof their armatures.

Assume that the temperature surrounding thermocouple 21 suddenly changesto a new and higher steady value. Due to the lag of the thermocouple thecurrent in the galvanometer 38 will increase gradually to a new value sothat the mirror turns its reflected light beam in a clockwise directionto position |22 on Fig. 1 in which the beam is completely shielded fromthe phototube. The photoelectric current instantly drops from l5 m. a.to 1 m. a. using the values of Fig. 3. The potential of grid 81gradually drops as capacitors 14 and 88 gradually discharge throughresistor 13 so that the grid potential gradually approaches thepotential in line 15 with the result that the minimum plate current owsthrough the coils of both relays and relay II also opens so that bothrelays are now open.

When relay II opens, it disconnects capacitor 88 from the governingsystem to greatly increase the rate of response and hence to decreasethe time-constant, which means that the time for a given response isconsiderably reduced, so that the governing system is much more livelythan before when capacitor 88 was connected with the same. In otherwords, the governing system is then quickly responsive to any changes ofillumnation so that the relay II can act very quickly following anincrease of illumination of the phototube.

The opening of relay II closes contacts 41 and 48 to connect point 55 tochange the point of connection of the negative side of thermocouple 21from its normally connected point 28 to point 55 of the potentiometercircuit with the result that the mirror galvanometer will be balancedsooner than it would if it were not for this change from point 28 to 55.The opening of relay II also opens contacts |I5 and IIl to leavecontacts IIS and of relay I2 closed so that coil |I4 of the reversingmotor II2 causes the motor to drive the sliding contact 4| upscaletowards its new position which corresponds with the new value of thetemperature of the thermocouple.

Shortly before the sliding contact reaches the proper position, thegalvanometer mirror swings the beam of light back in a counterclockwisedirection so that it is on the phototube. The photoelectric current owinitially increases Cross Heierence quickly to raise the grid potentialquickly and hence to build up the plate current quickly to a value whichcauses relay II to close thus closing contacts 84 and 85 to introducecapacitor 88 again into the governing circuit so that further changes ofillumination of the phototube cannot for a considerable time thereaftercause further operation of either relay II or relay I2.

The closing of relay II also shifts the point of connection of thenegative side of thermocouple 21 from point 55 back to the normal point28 which snaps the balancing point of the galvanometer from one position(which strongly tends to advance the galvanometer) back to its normalposition (in which the galvanometer is not pulled forward as hard as itwas before). The result is that, if the ir drop across resistor 23 besuficiently high, the galvanometer may be even kicked back, a situationwhich will not be considered immediately below but will be consideredfurther somewhat later herein.

Ordinarily the dead-zone is narrow and the galvanometer is so damped,which damping is proportional to its back E. M. F., that it will swingonly a little beyond the balance point due to the inertia of thegalvanometer, after which it will return asymptotically to its balancepoint assuming that, in the meantime, the motor |I2 has coasted to astop with slider 4| at the proper position. This motor is stopped in atleast approximately the proper position since the advancing E. M. F.advanced the closing of relay I I which closes contacts I I5 and IIBwhich short coil II3 so that both coils IIS and I|4 of the motor II2 arenow shorted which causes the motor to be very heavily braked to a stop.1f the slider 4I is not now at the proper position, the galvanometer 88will move slightly in the proper direction to cause a further additionalstep or so which will merely repeat the operation aforementioned if thestep be in the same direction.

However, if it be necessary for accurate measurement for the motor torun in the other direction, that is, due to an increase of temperatureof thermocouple 21, the operation is as follows: Starting with abalanced condition as before but with a sudden decrease of temperatureof the thermocouple, the potential of line 33 increases so that slidingcontact 4| must be moved downscale or to the left in Fig. 1 alongslidewire I8. The current flows through galvanometer 88 in a directionto cause the galvanometer to move the light beam in a counterclockwisedirection so that the illumination of the phototube and the relaycurrent are increased so that relay I2 closes, thus putting relays IIand I2 into the closed condition.

When relay I2 closes it opens contacts 8| and 82 to remove capacitor 88from the governing circuit and hence to increase the rate of response ofthe relay current to a change in the illumination and to decrease thetime-constant of the governing circuit so that the response time will bedecreased with the result that relay I2 will open very shortly after arebalancing of the galvanometer.

The closing of relay I2 also closes contacts 50 and 5| so that thebalance point is shifted from its normal point 28 to point 53, whichshift increases the force driving the galvanometer 38 back towards itsnormal position. The closing of relay I2 opens contacts IIE and |20 forcoil II4 which leaves coil I|3 alone eiective to drive Lxaminer lnotorH2 to move slider 4I to approach its new In the meantime, thegalvanometer hasstoppeditsnaturalswingandstartstoreturn toward atemporary position which is beyond the normal position and on theopposite sidefromthedirectionoftherwingbyan amount which depends uponthe ir drop across resistor 22. As the light beam traverses its normalposition, the ilhimination of phototube 6| now decreases so that thecurrent through the relays falls oil rapidly and relay l2 soon opens.

The opening of relay l2 closes contacts 9i and l2 to insert capacitor 8lagain into the governing circuit. This decreases the rate of responseoi' the governing circuit to further changes which greatly increases thetime-constant and the time of response to further changes and gives thegalvanometer a chance to settle down within the dead-zone without afurther operation of either relay. The opening of relay l2 openscontacts l! and 50 and at the same time closes contacts I and 5I toconnectI the negative side of thermocouple with the normal point 28 ofthe potentiometer circuit, so that the balancing point is shifted fromits position beyond the normal position back to its normal position. Theearly opening of relay I2 also closes contacts HS and t28 which shortcoil lil so that both coils are aborted and motor H2 is electricallybraked to a stop so that slider contact Il stops in atleastapproximately the right position.

In the foregoing, a complete cycle of operation has been followedthrough in a cursory manner for a change of temperature of thermocouple21 in each of opposite directions. However, the following description oithe operation is more exhaustive and hence more accurate especially asto the relative timing of changes of the temperature of thethermocouple, the position of the galvanometer, the position of theslider ll, and the condition of the governing circuit and of the relays.

In order to help in understanding the manner in which this controllingsystem operates, Fig. 3 is provided. Two cases are illustrated in thisligure. The ilrst case is hypothetical and shows the operation where thegoverning circuit is subjected only by the relays to the advancing E. M.F. by sluiting the point oi connection oi the thermocouple 21 from 58 to28 but not to change the time-constant of the governing circuit by theintroduction of capacitor 88 by the relays.

In the second case the relays act to subject the governing circuit bothto the advancing E. M. F. and to a change of the time-constant due tothe introduction of capacitor 88 by the relays when the value of therelay current corresponds with the balanced position of the light beamillustrated in Fig. l. In Fig. 3, the plate current of triode 68 and theposition of the median line of the light image in the plane of thecontrolling edge" are both plotted against time. Movement of the imageis shown in the nrst case along the solid line A--B-C with a suddenslight bend at B, and in the second case along the solid line A-B anddashed line B-C', these two curves being referred to the ordinatesidentified on the right. Corresponding variations in plate current areshown as the dotted line a--b-c the second case as the dotted line a-band the dot-dash line b-rL these two curv being referred to theordinates identified on the left. Instead of a single norxnal position,there is shown here to a greatly en- :some1 larged scale, 'a region ofrelay insensitivity or soealled dead-zone extending in both directionsfrom the controlling edge. The dead-zone of the image position of thebeam is referred to the ordinates identified on the right, and thedeadzone of plate current on a still larger scale is referred toordinates identified on the left and bounded by values of (e. g.) 10 and20 milliamperes. It is assumed in this iigure, for the sake of clarity,that when the relays operate there is no change in plate potential, andhence current, resulting from the introduction of the relay shuntresistors Nl and/or |04 into the circuit of Figure l.

Having identied and described the essentials or the circuits, let ussuppose that the E. M. F. oi the thermocouple 21 is less than thedifference in potential between points 28 and 42, or its contact 4I, inthe potentiometer and that the galvanometer 28 is deiiected so that thebeam of light is off the phototube 6l on the far side oi its controllingedge GI as shown at |22, Fig. l, stops on the galvanometer beingarranged as earlier noted to prevent it from deflecting the beam oil thephototube on the other side beyond the point (23. Since there is now nolight on the phototube, the plate current of triode tube $8 will be low,as at a, Fig. 3, as determined by sufiicient negative grid bias, and sothe plate current will be insullcient to close the relays. Coil IH o!the motor will be shortcircuited by contacts H6, |20 of relay I2 asdescribed and the motor then will be running in a direction to move thecontacts 4I down scale towards a value of E. M. F. diierence between l2and 28 which is equal to the E. M. F. of the thermocouple.

The deection oi the galvanometer from its normal position will now beproportional (neglecting inertia and damping factors) to the diiierencebetween the E. M. F. of the thermocouple and the potential differencebetween points I2 and i5, point 55 being now connected with thethermocouple by contacts 41 and 48 of relay Il. The galvanometerdeflection will consequently be less than it would be if thethermocouple were connected to point 28. Let us now suppose for thepurpose of further explanation that this deection is made less byexactly the equivalent of the back E. M. F., an effect obtained bymaking the if drop in resistor 23 equal to this value, and that relay Ilcloses instantaneously when the plate current reaches the closing valueoi 10 m. a. for example, and that the motor stops instantaneously withthe operation of relay Il which closes contacts H5 and H8 so that bothcoils III and H4 are shorted. Under such hypothetical circumstances,when the light beam reaches position B, Fig. 3, which position we willassume for the moment corresponds with the closing current of relay Il;this relay will close, the motor will stop and the thermocoupleconnection will be shifted from back to 28 by the opening of contacts 41and 48. An instant after these actions, the galvanometer will be at itsnormal position and the sliding contact Il will be at its properposition, the potential diilerence between points l2 and 28 will equalthe E. M. F. of the thermocouple and there will be no external net E. M.F. applied to the galvanometer.

I'here will, however, remain the back E. M. F. in the galvanometercircuit. The resulting current through the still moving galvanometertogecher with the restoring torque of its suspensions will serve tobring it to a stop as along the line BC. Having come to a stop at C, itwill presentlyswmgbacktothenm'malposionasisown along C-O while thereversing motor remains stationary provided that. during the intervalbetween the closing of relay il and the return of the galvanneter to itsnonna! pomtion, the plate currenthasbeensodelayedinitsrise.e.g. alongline b-d. as not to reach the closing current value of relay i2. If therise in the plate currmt had not been delayed by throwing capacitor B8into the grid circuit, but had maintained its rate along the line b-c,relay l2 would have closed. Now if we consider that time is necessaryfor relay II to close and that the motor will overrun after suchclosing, then it becomes apparent that the advancing E. M. F. should beincreased to compensate for these conditions.

Thus if the advancing E. M. F., i. e., the ir drop in resistor 23, ismade equal to the back E. M. F. plus the E. M. F. corresponding to theangle swung through by the galvanometer during the closing Y time ofrelay Ii plus the slide-wire E. M. F. covered by the motor during itsoverrun., then it is apparent that the motor will stop contact 4| at aposition l2 corresponding to the proper position as above and likewisethe galvanometer will finally come to rest at the same normal positionas described in the hypothetical instance. It is also apparent that themotor will remain stopped at such a position of l2 that the galvanometerwill come to a stop within the deadzone as along dashed line B-C'Oprovided the advancing E. M. F. was not so large that the galvanozneterwill be backed out of the deadzone nor so small that it will not swingback into it after having passed through it, for in the former caserelay il will reopen and in the latter case relay l2 will close.

While it may seem to have been implied that lthe dead-zone may have aconsiderable magnitude, its magnitude need not be large. In fact. it canbe made quite small, as will be clear from the following consideration.In the hypothetical case, the galvanometer may swing through quite anangle after relay il closes while it is being brought to rest mainly bythe retarding action of its generated back E. M. F. But in the realcase, the net E. M. F. acting upon the galvanometer at the instantfollowing the closing of relay Il may be much larger than the back E. M.F., being the sum of three parts as mentioned above withthe result thatthe retarding force on the galvanometer may be sufllcient to stop ltwithin a very small angle. During the brief interval of time for suchstopping, the overrun of the motor is also ilnished so that the motorand galvanometer come to rest at the same instant. This desirableperformance has been found by experiment and adjustment to be readilyattainable, and it has been found to be no larger necessary to applylarge forces to the relays and motor, or to incorporate auxiliaryrectifying devices in the balancing power circuit tospeeduptherelaysmdtoiocktbemotorwith direct current.

A fuller explanation oi the stepping @eration :lust mentioned is asfollows:

Assume that an increase in temperature of the thermocouple has occurredand that the galvanometer has denected the light beam in a clockwisedirection and the phototube current is so low that both relays are open.Capacitor 8B is disconnected from the governing system by opening ofcontacts 94 and $5 when relay Il opens. The point of connection on thenegative side of the thermocouple is shifted from its nor- Examinerapenas: t 9

maipdnt topointiibythe closing of contaots 4l and 48 when relay H opens.The motor t being driven upscale at full speed due to the opening ofcontacts H5 and H8 when relay il openswopencoil H3sothat onlycoil Ill isleft effective.

Also amume that the if drop across resistor 23 k so great that theclosing of relay Il when the iight beam reaches the dead-zone would, bythe removal of said if drop. cause the galvanometer to kick the lightbeam back out of the dead-zone which is what happens when the slider 4|approaches its proper position but has not quite reached it. In otherwords, when the light beam moves in a counterclockwise direction farenough, the phototube is sufficiently illuminated to cause relay il toclose.

The closing of relay il does three things: it includes capacitor I8 inthe governing system to delay the next operation of the relays, it actsto heavily brake motor H2, and it also acts to shift the point ofconnection of the negative side of the thermocouple from point 55 backto the normal point 28 with a potential change which is so great as toreverse the direction of the current through the galvanneter and causeit to kick the light beam back out of the dead-zone so that relay Ilopens as soon as it can in spite of the delay which accompanies theinclusion of capacitor I8 in the governing system.

Since the stopping of the motor was premature, the galvanometer iskicked back briefly from the same side of tbe dead-zone so that thelight. beam is soon again brought to within the dead-zone and meanwhilethe motor has taken a short step to move slider 4| a little further inthe same direction. Very soon after the light beam re-enters thedead-zone, the current in the governing system changes, since capacitor88 is still disconnected with the result that relay Il is reclosed withthe earlier described consequence. 'This stepping action may be repeatedseveral times although not more than a single step is acquired on theaverage in comercial prac- All of the above described actions resultsimilarly when the circuit of Fig. 2 is employed, in which the shift ofpotential of point 28 can be adjusted to the same magnitude as the ifdrop selected in resistor 23, Fig. l. It is apparent also that detailedconsideration would develop the fact that the performance in a reversedirection, in which the releasing time of relay I2 is included. insteadof the closing time of relay il, will arrive at the same qualitativeresults. Reasonable smallness of asymmetry of action is both desirableand attainable, as has been found also by experiment.

While in Fig. 3 and in the description of the perfomance of theapparatus, the assumption is made that there is no sudden change inplate current whenever relays Il or I2 act, there is actually such a.change as explained in the description of Fig. i. The sudden increasesin plate current when these relays close are of no signiilcance in theoperation of the apparatus provided the amplifier is sufciently powerfulto supply current substantially in excess of the requlrements. In otherwords, it is suiiicient if the closing current of -relay l2 issubstantially greater than the plate current observed the instant afterrelay Il clos and is also substantially less than the maximum platecurrent available.

The terms and expressions which I have emplo'yed are used as terms ofdescription and not of limitation, and I of such terms and equivalentsof the have no intention, in the use expressions, of excluding anyfeatures shown and described and portions thereof, but recognize thatvarious modifications are possible within the scope of the inventionclaimed. Where the word system" occurs in the claims, and especiallywhere a governing system is referred to, this is for the means stated tooperately connect the sensitive member with a final controlling elementwhich final element operates to perform a function related to the valueof the member-sensed variable and may be either an exhibiting means suchas an indicator, a pen or other marking device or may be a slidingcontact, a variable controllingresistor or any equivalents.

When a plurality of time-constants is referred to in the hereinafterappended claims, such term defines consecutively different magnitudes oia damping effect. The time-constant" refers to an effect, regardless ofhow it is produced, instead of to a cause. In other words, it is thetime-constant of a response of a condition such as, by way of example. arelay current to a change of illumination of a phototube. instead ofbeing limited to a product or ratio of values of resistance, inductanceand capacitance. The term time-constant of response has its usualmeaning: a rate of change of the response per unit oi theyet-unfulfilled portion of the response or, alternatively, the time itwould take the response to be complete if continued at the rate ofchange of the response at the instant considered. Mathematicailyexpressed, ior the response of current i to a sudden change ofillumination to have a single time-constant K, the current i must havethe following exponential relation with time t:

where llo is the ultimate current change and e is the Naperian base.While conventional instruments may be considered to have a plurality oftime-constants of response, these are not limited to clamping andsimultaneously, instead of consecutively, include either at least asecond timeconstant or a harmonic function, due to the inertia of theinstrument, having a generally unstabilizing eect.

I claim:

1. The steps in the method of controlling a member the position of whichis affected by an independent variable which is to be measured orcontrolled, which member governs a iinal element in accordance with thevalue of a physical condition of a governing system of an instrument ofthe measuring or controlling class, which steps comprise regulating thephysical condition in dependence upon the position of the member, andsimultaneously altering in accordance with the value of the conditionboth the time-constant oi the response of the condition to a change ofthe position of the member and the position of the element. in abalancing direction, to alter another variable reacting upon the member.

Z In measuring and controlling, in the nullmethod of altering onevariable voltage by a motor to balance another variable voltage which isto be measured or controlled, the dierence in voltages being indicatedby a galvanometer. the step oi' algebraically and continuously adding acorrectng voltage to one of said voltages according to the sign of theindicated difference of said voltages. said correcting voltage being ofa greater magnitude than that required to compensate for the back E. M.F. of the galvanometer and the momentum of the motor.

3. The steps in the method of stably controlling a measm'ing orcontrolling instrument which includes an initial member movable inaccordance with the value of an independent physical variable to bemeasured or controlled, a nal movable element to be controlled inaccordance said elements to govern the movements of the iinal element bythose of the initial member, which steps comprise regulating the valueof a in accordance initial member; in accordance with the value of saidphysical condition, both the ratio of the change in the value of saidphysical condition to the amount of movement of the initial member andthe eiiective time-constant of the response of the value of saidphysical condition to a change in the position oi' the initial member;and governing the moving of the ilnal element in accordance with thestated response of the physical condition of the governing system.

4. The method of controlling a measuring or controlling instrumentresponsive to an independently variable voltage which is to be measuredor controlled according to the position relative to a phototube of alight beam from a galvanometer, and in which a governing system includesa phototube and its amplifier whose output of amplified current dependsupon the relative positions of the light beam and phototube, and acontrolling means acting in response to the ampliiled current of thephototube and having a dead-zone which includes a null-position of thegalvanometer in which the light beam is partially on the phototube,which method comprises eontinuously influencing the position oi thegalvanometer according to the independently variable voltage, alteringthe time-constant oi the governing system to variably delay the responseof the amplified current to a change in the position of the light beam,and also influencing both the galvanometer and the stated alteration ofsaid time-constant of the governing system in accordance with the valueof the amplifled current of the phototube through the action of aportion of the controlling means which acts only in response to suchcurrent when such current is outside oi' the stated dead-zone to stablybring the light beam to rest at the stated nullposition.

5. In the method of controlling a member which is deilected from anormal position in accordance with changes in the value of anindependent variable which is to be measured or controlled, which memberacts upon a nal element through a governing system which operativelyconnects said member with said final element in accordance with thevalue of a physical condition of the governing system, and which systemin turn governs in accordance with the value of its condition aservomotor means for positioning said nnal element, the steps of varyingthe physical conditionv in response to the stated 'deflection of themember, and, when the condition substantially reaches a predeterminedvalue which ultimately corresponds with the normal position following adeilection of the member from its normal position and a return towardsaid position, sharply altering the timeconstant of response of thegoverning system to abruptly duce the rate of return of the conditiontoward said predetermined value.

6.Inanelectricalm'rcuitinameasin'ingor controlling instrument, thecombination of an amplifier of a variable electric Signal current, tworelays operable at different values of the amplified current, meansoperatively connected with said relays and said amplifier to alter theamplined current in a direction to reduce a departure o! the currentfrom the range between said values except when only one oi said relaysis closed, and means operatively connected with' said amplifier and withsaid relays to increase the time constant of response of the amplinedcurrent to a change of the signal current also when only one of saidrelays is closed.

7. In a measuring or controlling instrument oonnectable to a source ofvariable E. M. F., the combination of a reversible motor having means toactuate and to electrically brake the same, an electrical circuit havinga fixed point and including a slide-Wire. and a slide-wire contactmovable by said motor ior varying the E. M. F. of said movable contactrelative to that of said iixed point, a galvanometer and the source ofvariable E. M. F. connected between said xed point and said movablecontact, a mirror on said galvanometer movable in response to changes insaid independently variable E. M. F; and the E. M. F. of said movablecontact, said mirror galvanometer having a normal position correspondingto no current ow therethrough, a source of light, photoresponsive meansso arranged that a light beam reflected by said mirror is directedtowards said photoresponsive means, said galvanometer having a stoparranged relative to said mirror to keep the deflection of its reectedbeam upon said photoresponsive means when the beam tends to depart fromone side only, means for amplifying the current from saidphotoresponsive means, two relays connected with said amplifying meansand operable at dii'- ierent values oi the ampliiled current, saidrelays having contacts for controlling said motor, contacts operable bysaid relays for altering the poiential of said iixed point for affectingthe relation between the movements of said galvanometer and said motorin a balancing direction, other contacts operable by said relays, andmeans having appreciable capacity cooperatively connected by said othercontacts to retard the response of the amplified current to changes inthe position of the reflected light beam relative to saidphotoresponsivc means only while one set of relay contacts is open andthe other set closed, the motor-controlling contacts being arranged tobe connected to the braking means of the motor to cause the electricalbraking of the motor only while one set of relay contacts is open andthe other set is closed.

8. In a stable measuring or controlling in strument having a nal deviceincluding an element which is to be positioned in correspondence withthe value of an independent physical variable which is to be measured orcontrolled, the combination of a member which is continuously freelydeectable relative to a balancing position and is sensitive to saidvariable and to a balancing variable; a governing system, said memberbeing operatively connected with said t.

system to substantially continuously govern the value of a physicalcondition of said system over a range of positions including thebalancing position so that such value corresponds with the position ofsaid member over said range of 'MUSS nclol duw positions: mams forcontrolling said balancing variable by said element, and operativelyconnected with said member to aect the position of said member; meansconnected to the governing system for altering the time-constant ofresponse thereof to changes in the position of said member over saidrange of positions; and relay means operatively connected with' thegoverning system, with the controlling means, and with the response-ratealtering means, to be governed by the value of said condition of thegoverning system relative to that corresponding with the balancingposition of said member to eect the operation of the controlling meansto alter the balancing variable in a direction to balance theindependent variable and hence restore said member to the balancingposition, and to effect the operation of the response-rate alteringmeans so that the response-rate of said condition is slowed over a rangeof values of the condition of the governing system including that whichcorresponds with the balancing postion of said member.

9. In a measuring or controlling instrument, in combination, a membersensitive to a variable 'physical quantity, a device to be controlled bysaid member according to the sensed value of the variable, and anelectrical governing system including relay means controlled by saidmern ber and controlling said device, said system having elementsproviding a plurality of distinct time-constants consecutively effectivefor stabilizing the control of said device by said relay means.

10. In an instrument of the measuring and controlling class, thecombination of a primary member whose position is sensitive to avariable physical quantity, a nal device having an element whoseposition is to be controlled in ultimate correspondence with themember-sensed value for producing an operative effect in accordance withsaid value, and an electrical governing system in which the value of anelectrical condition is controlled by said member to tend to correspondwith the position of the member over a narrow range of positions and tohave a lower value of said condition with the member on one side of saidrange and a higher value of said condition with the member in anyoperating position on the other side of said range, and the value ofsaid electrical condition governs the stated positioning of the elementof said nal device to move said element when said value is outside ofsaid range in a direction to tend to return the member to within itsrange of positions, said system having elements providing a plurality ofdistinct time-constants effective to provide an initial Quick responseof the electrical condition followed by a slower response when the valueof the electiical condition reenters said range following a departuretherefrom, whereby stability of the podi the element in the final deviceis attained.

11. In an electrical instrument for measuring or controlling the valueof a variable electrical condition in a measuring circuit and includinga galvanometer connected with such circuit and including a light source,a mirror. and a phototube, all adapted and arranged to position thereflected image oi said light source relative to an edge of saidphototube in accordance with changes of value, and a circuit connectedwith said phototube to produce current therein in correspondence withthe pition of said image, the combination with said phototube circuit,

measuring circuit and galvanometer of: means having one delaying portionpermanently connected with said phototube circuit and another delayingportion connectable with said phototube circuit ior further delayingwith a distinct and different responsiveness the response of the currentin the phototube circuit to changes in said image position when thesecond named delaying portion is connected with said phototube circuit,means operatively connected with the measuring circuit for balancing thegalvanometer, and relay means functioning in dependence upon saidcurrent in the phototube circuit to govern the operative connection oithe last named portion of the delaying means with said phototube circuitand to govern the operation oi' the balancing means.

12. The combination set forth in claim 11 in which said relay meanscomprises two relays having their coils connected with said phototubecircuit, said relays having contacts operative at different currentvalues in said phototube circuit, and said relays and their contactsbeing adapted and arranged to operate the balancing means only when thevalue of said current is outside oi the stated different current valuesand to connect the second named portion of the current-response delayingmeans with said phototube circuit only when the current in the phototubecircuit is between said diiierent contactoperating values.

13. In an electrical instrument of the measuring and controlling classwhich is connectable with a source oi' a variable physical quantity. thecombination of a primary member sensitive to the variable, a iinaldevice reactinguponsaid member and to be controlled by said memberaccording to the member-sensed value of the variable, an electricalgoverning system for the device including a means operatively connectedto the primary member to cause a current in said system to be responsiveto the member-sensed value of the variable, relay means operable outsideof a zone bounded by two diilerent values of the current, said relaymeans being operatively connected to the final device to cause theoperation of said device in a direction to reduce a departure of thecurrent from the zone between its said values only when the value o! thecurrent as determined by the relay means is outside of said zone, andmeans operatively connected with said system and with said relay meansto decrease the time-constant of the response of the relay-operatingcurrent to a change in the member-sensed value of the variable also onlywhen the value of the current as determined by the relay means isoutside of said zone.

14. In an instrument which is connectable to a source of a physicalvariable to be measured or controlled, the combination of a membercontinuously positionable in response to changes in the value oi thevariable, a governing system comprising at least a relay meanscontrolled by and responsively sensitive to the position of said memberrelative to a xed normal position upon a change in said value, areversible motor governed by said relay means, a controlling devicehaving an element operated by said motor to return said member to a ixednormal position upon a change in said value, means also governed by saidrelay means to advance the phase o! said member relative to that oi saidelement, and means for retarding the action oi said relay means actuatedby said relay means and reacting upon said relay means to increase theretarding action when said member is near its normal position as sensedby said relay means, said relay means being also operatively connectedwith said motor to cause braking thereof when said member is near itsnormal position as sensed by said relay means.

15. In an instrument of the measuring and controlling class which isattachable to a source 0! an electrical variable to be measured orcontrolled, the combination of a ilrst circuit which is attachable tosaid source, a galvanometer connected with said circuit and having amember which is movable in accordance with a variable current in thegalvanometer portion of said circuit from a normal positioncorresponding with zero current through said circuit portion,compensating means connected with said circuit for altering the currenttherein, a reversible motor having appreciable inertia and connected tosaid compensating means to operate the latter, a second circuit,current-modifying means connected with said second circuit and operatedby said galvanometer member to modify the current in said second circuitin correspondence with the position of said member whenever said memberis in a position to cause such modification. and relay means operativelyconnected with said second circuit and continuously sensitive to thecurrent therein, said relay means being operatively connected also withsaid motor to govern the actuation thereof to reduce the departure ofsaid member from its normal position only when the current in the secondcircuit is outside of a deadzone limited by two values of the current inthe second named circuit which are close to and respectively above andbelow said normal value and to act to cause braking of said operatingmeans when such current is within said dead zone, and said galvanometermember being continuously free to move both at positions correspondingwith said current dead-zone and in adjacent current-modifying positions.

16. In a governing system of a measuring or controlling instrument, incombination, an illuminated mirror galvanometer which is continuouslyfree to deilect from its null-position upon a difference between anindependent variable to be measured or controlled and a balancingvariable, a phototube located in the path of a light beam from themirror which deects when the galvanometer senses said difference andwith the beam only partly upon the phototube in the nullposition oi' thegalvanometer, means for controlling the balancing variable to rebalancethe galvanometer, a relay means connected with and continuouslycontrolled by current from said phototube and operatively connected withthe controlling means to govern the balancing of the two variables whenthe current is outside of a zone including the value corresponding withthe null-position of the galvanometer, and damping means operativelyconnected with and controlled by said relay means to slow the rate o!response of the relay means to a change of the position of the lightbeam sensed by said phototube when the beam is within a zone ofpositions including the null-position, whereby continuing oscillationsof the galvanometer tend to be inhibited or damped out by the dampingmeans when the light beam is only partially on said phototube so thatthe value of the balancing variable is stably brought into ultimatecorrespondence with the value of the independent variable.

17. Measuring apparatus including means for impinging a beam o! light ona lightsensitive element, a continuously-positionabie means forproducing deections of said light beam from a wedetermmed normalposition in accordance with variations in a condition to be measured,means for shifting the position of balance-of the second named means bya nite amount from said normal position, and means responsive to theconductivity oi said light-sensitive element adapted to control the saidcontinuously-positionabie means for gradually reducing the deflectionsof said beam from said normal position as long as the response ot saidresponsive means indicates that the light beam is appreciabiy deliectedfrom its normal position, the balanceshifting means being under controloi said responsive means to operate to temporarily reduce the eiiect oisuch deiiection only as long as the stated response indicates that thelight beam is appreciably deflected from its normal position.

18. In a stable instrument which is connectable with a source of anelectrical variable to be measured or controlled, the combination of acircuit having a slide-wire and a shunting reaistor therefor in which anoperating current iiows to produce potential differences at diflerentpoints along the slide-wire and shunting resistor, said circuit alsoincluding a contact which is slidable along the slide-wire; agalvanometer having a movable member and two terminals of which one isconnectable with one side of the source of the electrical variable andthe other with the sliding contact, a governing system including a relaymeans in which a current is responsively sensitive to the position ofthe galvanometer member when the member is outside ci a dead-zone of therelay means, which dead- -zone includes the null-position of the member,

and a motor for moving the sliding contact at substantially constantspeed as long as the relay means is effective, said relay means beingarranged to selectively connect the other side of said source withpoints in said circuit having substantially the s ame potential aspoints so widely spaced along the shunting resistor as to apply by theoperation of the relay means such a large galvanometer-advancingconstant voltage upon leaving the dead-zone that, upon reentering thedead-zone, the galvanometer will be repeatedly kicked back from itsposition at the instant of the removal of such voltage to again applythe advancing voltage to the system, whereby in rebalancing the slidingcontact will proceed to take short steps towards a position whichcorresponds with the value of the variable being measured.

19. In a measuring or controlling instrument having a nal controlleddevice including a movable element which is to be positioned incorrespondence with the value of an independent physical variable to bemeasured or controlled,v

the combination of a deiiectable member which is sensitive to saidvariable, a governing system in which the value of a physical conditionis responsively sensitive to the deilection of said member, relay meansoperatively connected with said system to operate when a dead-zoneenclosed by two predetermined values of said condition is entered orleft by such condition in either direction, a reversible motor having acoil for each direction of rotation which is operatively connected withsaid relay means and with said element to govern the operation oi theele- 'Cross Reierence maar is outside said dead-zone, and means operatedby the relay means to short-circuit both of said coils substantially aslong as the operation oi the relay means indicates that the value of thecondition is within said dead-zone.

20. In a measuring or controlling instrument having a final controlleddevice including a movable element 4l which is to be positioned incorrespondence with tbe value of an electrical variable to be measuredor controlled in a circuit including a source 21 of said variable, anelectrical system (including 6l) for governing the positioning of theilnal movable element Il in accordance with the value oi an electricalcondition of the system, a galvanometer 38 member which is sensitive tosaid variable and operatively connected with said system (through Il) togovern the value of said condition of said system, relay means H and i2operatively connected with said system to operate when a dead-zoneenclosed by two predetermined values o! said condition is entered orleft by such condition in either direction, a reversible motor H2 whichis operatively connected with said relay means Ii and i2 and with saidnnal element Il to govern the operation of the element 4| in a directiontoward the stated correspondence only when the condition of the relaymeans indicates that the value of the condition is outside saiddead-zone, the combination oi: means 22 and 23 in Fig. 1 operativelyconnected with said relay means il and i2 and with said galvanometer 38member and operated by said relay means il and I2 to alter therelay-operating positions oi' the galvanometer 88 member by apredetermined amount to cause earlier relay operation subsequent toreversal of direction of said member than would occur without suchaltering of the position of said member, and means including a capacitor88 and connectabie with the governing system and operatively connectedwith the relay means tov be operated by the relay means to sharplyincrease the time-constant of the response oi said electrical conditionupon a change in the position of the galvanometer when the value of saidcondition enters the dead-zone as indicated by the operation of therelay means.

21. An instrument of the measuring and controlling class comprising, incombination, an initial member moved in accordance with the value of aphysical variable to be measured or controlled, a iinal movable elementand a reversible motor for moving said element, and a governing systemoperatively connecting said member and said final element to govern themovements of said nal element in accordance with those oi the initialmember, said system comprising: a ilrst means operated by the member toregulate the value oi a physical condition of said system in accordancewith the position of the member relative to a normal position of themember, a second means for altering the sensitivity or ratio of thechange in the value of the physical condition to the amount of movementoi the member, a third means for altering the time-constant of responseof the value of said physical condition to provide a plurality ofdistinct time-constants of delayed response oi' said physical condition.and a fourth ment in a direction towards the stated corremeans sensitiveto the value of the delayed response of said physical condition andoperated Examine,r

the motor, the second means and the third means to operate the motorwhen said value is outside of said zone to move the final element to aposition which depends upon that of said member. to operate the secondmeans when the said zone is re-entered by said value following adeparture therefrom to then provide an at least initial lowering of thesensitivity, and to operate the third means to provide an increasedvalue of said time-constant when the operation of the fourth meansindicates that said value is within said zone.

22. A governing system for a controlled device comprising, incombination, a circuit which is connectable with a source oi' currentsupply, means connected with the circuit for modifying the current inthe circuit from a normal value in response to departures of the valueoi a physical variable with which the value oi the current in thecircuit ultimately corresponds from a predetermined value which is setto correspond with said normal value of the current in said circuit, arelay means connected with the circuit and actuated by the currenttherein upon a departure from or return to a zone of circuit currentvalues which includes said normal value, a means for controlling saiddevice operatively connected with the relay means to be governed therebywhen the operation of the relay means indicates that the current in saidcircuit is outside of said zone, and means connected with the circuit bythe relay means for abruptly altering the response of the current to thestated changes when said relay means operates to then cause said circuitto have two complementally effective distinct time-constants of responsewith the time-constants oi' a different order of magnitude and with theslower response effective within said zone of current in said circuit.

23. A governing system comprising, in combination, a circuit which isconnected with a source oi current supply, photoresponsive meansconnected with the circuit for modifying the current therein from anormal value in response to changes in its illumination, a means ioraltering said illumination in accordance with changes in the value oi' aphysical condition, a motor, means operated by the motor to restore theillumination oi the photoresponsive means to said normal value, a relaymeans connected with the circuit and actuated by the current therein andwith said motor to govern the operation thereof, and means connectedwith the circuit by the relay means for damping the response of thecurrent to the stated changes to cause said circuit to have twocomplementally effective distinct time-constants of said response withthe time-constants of a diierent order of magnitude.

24. The combination set forth in claim 23 in which said relay meanscomprises two relays having their coils connected with said circuit.said relays having contacts for making the stated connections and beingconstructed to operate said contacts at different values of the currentwhich values establish a zone that includes the value that ultimatelycorresponds with the normal illumination, and said relays beingconstructed and arranged to abruptly connect the damping means to thecircuit when said current is within said zone to then slow the responseof said current to changes of said illumination and to govern theoperation of the motor when said current is outside said zone.

CHARLES O. FAIRCHILD.

